The blocking of polyisocyanates for temporarily protecting the isocyanate groups is a long-known operation and is described for example in Houben Weyl, Methoden der organischen Chemie XIV/2, pp. 61–70. Curable compositions comprising blocked polyisocyanates find use for example in polyurethane coating materials.
An overview of blocking agents suitable in principle is found for example in Wicks et al. in Progress in Organic Coatings 1975, 3, pp. 73–79, 1981, 9, pp. 3–28 and 1999, 36, pp. 148–172.
In aqueous coating compositions it is common to use hydrophilicized blocked polyisocyanates, whose preparation is described for example in DE-A 24 56 469 and DE-A 28 53 937.
A disadvantage when using prior art hydrophilicized blocked polyisocyanates is that after the deblocking and/or crosslinking a certain fraction of the blocking agent remains in the resultant coating film and adversely affects its quality.
Qualities such as scratch resistance and acid stability of one-component coating films are poorer because of the remanant blocking agent than those of two-component (2K) polyurethane coatings (e.g. T. Engbert, E. König, E. Jürgens, Farbe & Lack, Curt R. Vincentz Verlag, Hanover October 1995). The elimination of the blocking agent and its gaseous escape from the coating film can lead to blistering in the coating. Subsequent incineration of the emitted blocking agent may possibly be necessary from environmental and occupational hygiene standpoints.
In systems including the prior art hydrophilicized blocked polyisocyanates, the baking temperatures are typically from 150 to 170° C.
For aqueous 1K coating systems with lower baking temperatures of 90–120° C. polyisocyanates blocked predominantly with diethyl malonate have recently found use (e.g. EP-A 0 947 531). In contrast to blockings with, say, N-heterocyclic compounds, such as caprolactam or dimethylpyrazole, or else with butanone oxime, for example, the blocking agent in this case is not completely eliminated: instead there is a transesterification on the diethyl-malonate-blocked isocyanate with elimination of ethanol.